From a geometrical point of view, there are two types of methods for minimizing the losses in a curved path length of an optical guide.
One first method was described in detail in the doctorate thesis of E. C. M. Pennings and entitled "Bends in Optical Ridge Waveguides, Modeling and Experiments", Technische Universiteit, Delft, Holland, 1990.
It consists of using curved paths along arcs of circles. A path is then obtained with a constant curvature per portions and the losses per curve can easily be calculated. Moreover, when observing that the luminous intensity maximum of the fundamental mode of the curved guide is offset towards the outer edge of the guide and that its distribution is slightly flattened out, it is possible on each sudden change of curve to offset the centers of the guides so as to recenter the propagation modes and thus reduce the transition losses between two curve discontinuities.
Secondly, it is known that the losses per curve reduce when the width of the guide increases up to a certain limit value, from which a "whispering gallery mode" is reached. Thus, it is possible to improve the losses of the short path by enlarging the guide to this width. The expression "width of the guide"' is understood to be the width of the portion of the guiding structure which ensures the lateral containment of the light; for example, the width of the etched film (i.e. guiding film or adjacent film) in the cases described earlier.
By means of this first method, it is thus possible to obtain a guide with curved portions with a constant width for each portion with an offsetting between the centers of the two neighbouring portions.
A second method consists of using a path length with a continually variable curve, said method being described in the doctorate thesis of F. Ladouceur and entitled "Buried Channel Waveguides and Devices", Australian National University, Canberra, Australia, 1992. Since the sudden curve changes cause transition losses between the various portions of the path, a continually variable curved path length ought to have greater effectiveness as regards transmission of the luminous power.
The path length of the guide is selected from a family of inparametric curves ##EQU1## 0&lt;t&lt;L, the choice of the path length being finally effected by the minimization of a functional.
It ought to be mentioned that this method does not provide for any modification of the width of the guide along the path length.
The major drawback of the first method is that the minimization of losses is carried out by introducing discontinuities which generate transition losses and possibly partial reflections. Therefore, it is intrinsically limited. Secondly, the embodiment of optical guides generally passes through a masking photolithography stage and thus requires that the structure be correctly described on the mask. Now, the embodiment with sufficient accuracy of this type of path length is delicate for a masking device and the method is extremely sensitive to any embodiment imperfection.
The second method is better in that it is generally more flexible and easier to carry out, but as it stands at the current moment, has limited effectiveness in terms of a reduction of the optical losses.